Generally speaking, there are two methods by which the motorboat impeller is fastened with the motorboat driveshaft. The first method includes a impeller which is provided centrally with a threaded axial hole engageable with a threaded portion located at the tail end of a driveshaft. The second method includes a impeller which is provided centrally with an axial hole having therein a predetermined number of serrated slots which extend along the direction of the axis of the axial hole and which are engageable with splines disposed on the tail end of a driveshaft. The motorboat impeller of the present invention relates to the first fastening method described above.
As shown in FIGS. 1 and 2, a conventional motorboat impeller 10 of the prior art comprises an axial portion 12 and three blades 14 fastened to the periphery of the axial portion 12. Located at the center of the axial portion 12 is an axial hole 16 extending through the axial portion 12. The axial hole 16 is provided in the rear end thereof with a female threaded portion 17 engageable with a male threaded portion located at the tail end of a driveshaft (not shown in the drawings). The axial portion 12 has a smooth external surface, which can not be held firmly with a tool at such time when the work of fastening the impeller 10 with the driveshaft is under way. For this reason, the prior art motorboat impeller 10 is provided with a hexagonal portion 13 contiguous to the axial portion 12 and extending along the direction of the axis of the impeller 10.
As shown in FIG. 2, the hexagonal portion 13 has six sides 131, which can be held firmly with an appropriate wrench to facilitate the work of fastening the impeller 10 with the driveshaft. However, the prior art motorboat impeller 10 described above is defective in design in that the hexagonal portion 13 has six corners 132, which are responsible for the formation of water bubbles, turbulent waves and cavitation at the time when the impeller 10 is driven by the driveshaft to turn at a high speed in the water. In other words, the operation of the prior art motorboat impeller 10 can bring about a severe vibration of the motorboat and a substantial reduction in the thrust of the impeller 10.